Those critics who have assumed that the "religious aspects" of the books are the most "schematic" themselves rely on a rather schematic assumption about artistic worth. The truth is, the books are bad where Lewis' skills as a writer and a thinker fail him. In some places, it's where the plot creaks like an old suit of armor, because plot isn't what mattered most to him, and in others, it's where he let his didactic, expository, Anglican self get the better of the spell-caster within.
In the end, Narnia is not a stand-in or merely an "allegory" for our world. It is, quite explicitly, an alternative to it, complete with its own pleasures and typologies. Interestingly enough, among the testimonials to The Chronicles on a Christian Web site, there are plenty from kids who praise the books' "adventuresome" qualities, while the few who talk about the book's Christian themes note that it was their mom or church leader who alerted them to the connection—at which point they "saw" it. That would probably have pleased Lewis, who once wrote of his distaste for critics who allegorize texts, and whose curious allegiance to both "deep magic" and Christianity would presumably have caused him to balk at the reductive debate unfolding among sensible adults today.